The invention concerns an electronic circuit having an on-and-off time hysteresis for controlling the operation of a boat bilge pump, and having two probes which sense water level in the bilge.
Typically, boat bilge pump control systems consist of an impellar-type DC water pump, controlled by either a manual switch or a mechanical switch activated by a float mechanism. As the water level in the bilge rises, the float naturally rises until the float reaches a pre-determined point, and a mechanical switch is activated to turn on the pump. The switch remains in the on position and the motor active until the water level in the bilge drops to a second pre-determined point to cause the mechanical switch to disengage and turn off the motor. In such systems, the switch per se is unreliable due to contact degradation caused by switching an inductive load. Float-activated systems may be become sticky in operation due to small pieces of debris and contaminants, such as seaweed, sand, dirt, wood chips and silt which restrict the motion of the mechanical system. Since failure of the switch or float can result in the boat sinking, (for example, when unattended at a dock or slip) it is common practice to replace the switch and float annually. Liquid mercury switches are also available, which are more reliable, but are more costly.
U.S. Pat. No. 4,222,711 and 4,357,131 disclose the use of two electrodes for sensing a high-water level and to regulate the actuation of a pump. A timing circuit is provided so that the pump is actuated for a pre-determined time before it is shut off. The teaching of multiple-actuation pumps as regulated by circuits having complex configurations of probes, is separately found in U.S. Pat. Nos. 3,509,825; 3,787,733; and 4,105,367.